Numerous designs of hand-held light measuring devices are known. They are usually designed as densitometers or color measuring devices, and various measuring technologies are used. Many known hand-held light measuring devices are designed as spectral photometers and can therefore be used universally.
The known light measuring devices usually comprise an arrangement for illuminating the measurement object, a pick-up arrangement for detecting the measurement light reflected back from the measurement object, a photoelectric converter arrangement for converting the detected measurement light into corresponding electric signals and an electronic system for evaluating the electric signals and for controlling the operating sequences of the measuring device. The lighting arrangement may generate white light or may be designed for colored light. The converter arrangement may comprise a number of color filters or dispersive elements, e.g. a diffraction grating, for splitting the measurement light into different wavelength ranges. Photodiodes or CCD converter arrangements may be provided for the actual conversion into electric signals.
To measure a measurement object, the hand-held light measuring device is placed on the point of interest (measurement site) of the measurement object, and the measurement is taken through a measuring orifice disposed in the bottom face of the device housing.
What all of the known light measuring devices have in common is that they have to be calibrated from time to time. They are often equipped with an integrated white reference tile for this purpose, which can be introduced into the measurement optical path for calibration purposes. When using the device as normal for taking measurements, the white reference tile is disposed in a parked position outside of the measurement optical path. The white reference tile may be disposed on a support element which is able to slide or pivot in the device housing, for example. Also already known are light measuring devices where the measurement optical path is deflected onto a stationary mounted white reference tile using mirror elements which can be moved into it and moved back out of it again. Alternatively, the white reference tile may be provided as a separate component which takes the place of a measurement object for calibration purposes.